1. Field of the Invention
The invention relates to analog-to-digital converters (ADC), and more particularly to an analog-to-digital converter comprising delta-sigma modulators.
2. Description of the Related Art
Delta-sigma modulation is a popular method for high resolution analog-to-digital conversion. It is frequently used in audio processing and has a wide rage of applications. Because most audio signals are presently transmitted, processed, and stored in digital form, analog signals must be converted to digital signals for further processing. When an analog signal is converted to digital form, a one-bit data stream is desirable in many applications because there is a one-to-one correspondence between the input clock and the output data. The number of signals required for a system to communicate with the ADC is two. Thus, delta-sigma modulation is widely used.
FIG. 1 is a block diagram of a portion of an audio processing device 100, which includes an conventional analog-to-digital converter 108. The audio processing device 100 includes a microphone module 102, the analog-to-digital converter 108, and a digital signal processing (DSP) module 110. The microphone module 102 receives the external sound wave and converts it to an analog signal. The DSP module 110 is the core of the audio processing device 100 and undertakes the major load of audio processing tasks. Because the DSP module 110 can only handle digital signals, the analog signal must be transformed to digital format. The analog signal is delivered to the analog-to-digital converter 108 and further converted to a medium frequency datastream, which is used as the digital input signal of the DSP module 110 in substitution for the analog signal.
The analog-to-digital converter 108 includes a delta-sigma modulator 104 and a decimator 106. The analog signal is first converted by the delta-sigma modulator 104 to a high-frequency datastream. The high-frequency datastream is then decimated by the decimator to generate the medium-frequency datastream, which is the output of the analog-to-digital converter 108. “Decimation” indicates periodically dropping the undesired samples of the high-frequency datastream according to a decimating factor, so that the remnant samples of the high-frequency datastream meet a desirable sampling rate, wherein the decimating factor is given by the quotient between the sampling rate of input and output signals.
The analog-to-digital converter 108 of FIG. 1, however, lacks the ability to generate multiple outputs of different phases. When the DSP module 110 requires input signals of different phases for further processing, the DSP module 110 has to transform the medium-frequency datastream to the different phase signals itself, wasting processing resources of the DSP module 110 and complicating the design of the DSP module 110. If the analog-to-digital converter 108 can directly generate the different phase output signals according to the analog signal, the processing load of the DSP module 110 can be reduced. In addition, since the sampling frequency of the plurality of low-frequency datastreams are less than that of the medium-frequency datastream, the DSP module 110 can operate at a lower frequency, and the power consumption of the system can be reduced. Furthermore, if not all samples of the medium-frequency datastream is required in some situations according to the requirements of the DSP module 110, the medium-frequency datastream samples of predetermined phase can be extracted by the analog-to-digital converter 108 to generate only the desired datastreams required by the DSP module 110.